Magnetic resonance imaging apparatus of the above type are already known in the art. In prior art apparatus, the patient is placed on a patient table or a support structure, to which he/she/it is secured and by which he/she/it is displaced relative to the chamber or the cavity.
It is known that the magnet structure comprises means for generating a static magnetic field in the chamber or the cavity. It is also known that the static magnetic field must have a high homogeneity in the area coincident with that of which valid, i.e. useful images are desired. Therefore, since the openings of the magnet structure naturally generate distortions of the static field, such field cannot have the necessary or required characteristics for obtaining useful or valid images throughout the volume of the chamber, its validity being restricted to a smaller volume portion of the complete volume of the chamber. The said portion of the volume of the chamber which is permeated by the static field having the desired qualities for imaging is generally known as imaging volume, and is delimited by an ideal spherical surface.
Therefore, the imaging volume is a portion of the entire cavity or chamber volume, delimited by the surface of an ideal sphere, within which spherical surface the static field has the homogeneity required for magnetic resonance imaging.
In addition to the simple spherical shape, ellipsoidal surfaces or combinations of ellipsoidal and spherical surfaces having variously inclined axes may be used as ideal delimiting surfaces for the volume permeated by the magnetic field having the homogeneity required for magnetic resonance imaging.
Two opposite approaches are used in prior art apparatus. On the one hand, there is the tendency to make very large magnet structures, having a large cavity, wherefore the imaging volume is also large and may envelope and extend over several anatomic regions of a human or animal patient body. On the other hand, a slice-like imaging volume is provided, whereby one slice at a time is imaged, like in tomography.
In the former case, the magnet structure is very large and designed to accommodate the whole patient therein. This solution is suitable, although expensive, for human patients or small animals, but causes problems, when used in veterinary applications, if the patient to be treated has a relatively large size, such as a horse or the like.
Furthermore, particular means, as well as a certain effort are needed for positioning an animal patient on a support structure in an unnatural position, and in some cases the animal must be even anesthetized or narcotized to be secured to the structure. This requires the use of drugs and a long and difficult imaging procedure, as well as special bulky means for positioning, displacing and safely holding the patient to the support structure, particularly when the patient is an animal.
The other approach, that uses very small magnet structures to generate an image as a sum of images of slices of an anatomic region, requires long scan times, wherefore interventions, arrangements and devices are needed to hold at least the patient body part to be imaged in a stable and fixed position. If the patient is an animal, possibly of relatively large size, then the devices required to hold at least the relevant anatomic region of the animal in position for a long time are definitely bulky, hence the magnet structure shall be accordingly adapted to accommodate at least a portion of the patient support device, wherefore the magnet structure shall also have a relatively large size, which neutralizes the advantage provided by the small size of the magnet structure used for multi-slice imaging.